The initial characteristics stem from WO-A1-00/02825, which describes a method for marking glass panes after a heat treatment. According to a preferred application case mentioned in this description, tempered glass panes, that have undergone an aging test following the tempering operation, are provided with a local colored marking. This marking is produced by means of a special thermochromic organic color, that is applied locally, for example by screen printing, after the tempering but before the aging at a predetermined point on the surface of the pane, or alternatively on a thin film covering the latter.
It has been shown that, using the heat soaking test, tempered glass panes do not contain critical nickel sulfide inclusions. It is known that such inclusions can lead to sudden spontaneous fracture, with unpredictable consequences, during the life of the tempered glass panes. During the heat soak test, in which the panes are heated to maximum temperatures of generally between 180 and 340° C., especially around 300° C., according to a predetermined time-temperature curve, panes break in a random fashion before they are mounted. The tempering of the panes has still not disappeared at these temperatures. In any case, this heat soaking test takes a great deal of time and necessarily incurs relatively high installation costs.
If the thermochromic color has been applied before the test, it undergoes a permanent change by an irreversible conversion of the color. Consequently, it is possible immediately to detect that the heat soaking test has been carried out on the completed, heat-tested/treated, tempered glass panes. Even very small residues of color may be clearly identified, by suitable methods, after the heat soaking test has been carried out.
For the application described here, thermochromic colors that contain the chemical compound iron (II, III) hexacyanoferrate as thermochromic pigment may be suitable. An example of such a commercially available pigment is “Mannox Blue 510”. The color must exhibit good adhesion to the glass because of its composition.
Colored markings of this kind cannot be compared with inter alia an enamel. They cannot penetrate the surface of the glass or bond in a lasting manner thereto. On the contrary, it is possible in principle to remove them from the surface of the glass, completely and virtually without any trace, using a blade or steel wool after a heat soaking test. As a result, no reliable identification of the tested panes is possible or, put another way, it is not possible to exclude with certainty the fact that panes with no color mark have not undergone a heat soaking test. Thus, the indicative value of the color mark and the visible evidence of quality that stems therefrom are undesirably limited.
In addition, it should however not be possible to apply the marking color before the tempering, and produce the color change only by dint of the thermal tempering, without subsequently carrying out a heat soak test. The aforementioned color loses all adhesion to the surface of the glass after the action of high temperatures substantially above 300° C., such as those necessary for tempering the glass (well above 600° C.), so that they can no longer be used for the marking.
Also known, from document DE-C1-3 940 749, is a method for the durable marking or printing of glass panes, in which, in a layer deposited by screen printing, local color changes are induced via which the individual characteristics of the pane in question are indelibly displayed. This occurs in the known method by the fact that a marking composed of an organic material (color or ink) is applied locally, in a predetermined design, on the surface of the pane before printing with the inorganic screen-printing paste. During the high-temperature baking of the subsequently applied screen-printing paste, the organic compounds volatilize. Thus, local perturbations, or even holes, appear in the organic coating and these are clearly perceptible visually. Modification or removal of these designs is possible only by simultaneously removing the baked inorganic coating.
It is also known (DE-C2-4 111 625 and EP-B1-0 281 351) to produce fine structures, in the form of striations or similar features, in coatings deposited on a substrate by screen printing. In the aforementioned cases, these local heterogeneities serve to improve the brazability of the screen-printing paste that is necessarily electrically conducting after the baking operation, or alternatively to improve the adhesion of the spots of braze.